https://chemistry.beloit.edu/classes/nanotech/ZnO/mattoday10_05_40.pdf

ZnO - nanostructures, defects, and devices

This paper deals with the application of ZnO varistors—an area which has not been treated in a systematic way in the literature. The paper starts with a brief description of the fundamental properties comprising the electrical behavior as well as the physics, chemistry, and microstructure of the varistor. These properties then form the basis for defining the application parameters that are directly related to the nonlinear current-voltage characteristics of the varistor. This paper provides a detailed description of these parameters and their relation to microstructure and the processing of the varistor. Finally, a discussion is presented on the reliability of the varistor by considering a grain-boundary defect model which explains both the instability and the stability under use conditions.

Application of Zinc Oxide Varistors

ZnO nanoparticles were prepared by coprecipitation method at 450C. X-ray diffraction result indicates that the sample is having a crystalline wurtzite phase. Transmission electron microscopy (TEM) result reveals that the ZnO sample is spherical in shape with an average grain size of about 50nm. X-ray peak broadening analysis was used to evaluate the crystalline sizes and lattice strain by the Williamson-Hall (W-H) analysis. All other relevant physical parameters such as strain, stress, and energy density values were also calculated using W-H analysis with different models, viz, uniform deformation model, uniform deformation stress model and uniform deformation energy density model. The root mean square strain was determined from the interplanar spacing and strain estimated from the three models. The three models yield different strain values; it may be due to the anisotropic nature of the material. The mean particle size of ZnO nanoparticles estimated from TEM analysis, Scherrers formula and W-H analysis is highly intercorrelated.

/pdf/williamson-hall-analysis-in-estimation-of-lattice-strain-in-1epz6r1apj.pdf

Williamson-Hall analysis in estimation of lattice strain in nanometer-sized ZnO particles

Three different values (3.1, 3.2, and 3.3 eV) have been reported for the optical band gap of zinc oxide single crystals at room temperature. By comparing the optical properties of ZnO crystals using a variety of optical techniques it is concluded that the room temperature band gap is 3.3 eV and that the other values are attributable to a valence band-donor transition at ∼3.15 eV that can dominate the optical absorption when the bulk of a single crystal is probed.

On the optical band gap of zinc oxide

ZnO nanoparticles in the size range from 2 to 7 nm were prepared by addition of LiOH to an ethanolic zinc acetate solution. This method [Spanhel, L.; Anderson, M. A. J. Am. Chem. Soc. 1991, 113, 2826] was modified and extended at several points. The synthesis of very small ZnO nuclei was simplified. It was found that aging of particles was governed by temperature, the water content, and the presence of reaction products. Water and acetate induced considerably accelerated particle growth. Growth could almost be stopped by removal of these species (“washing”). Washing consisted of repeated precipitation of ZnO by addition of alkanes such as heptane, removal of the supernatant, and redispersion in ethanol. The aging characteristics are interpreted in terms of the concentration of dissolved ZnII species and reactions well-known in sol−gel chemistry. These findings present a better-defined and more versatile procedure for production of clean ZnO sols of readily adjustable particle size. Such sols are of partic...

Synthesis and Growth of ZnO Nanoparticles

Linear shrinkage profiles of an unconstrained gold thick film material and a low -tempearture cofireable glass-ceramic (LTCC) green tape were measured using a noncontact optical technique. A laser beam scans across a sample, at various times during the sintering process. The unconstrained sintering kinetics of the gold film were found to differ significantly from those of the LTCC tape. The densification of the gold film was nearly completed before the LTCC began to densify. The development of camber (warpage) during consintering of a gold/LTCC composite structure was monitored and recorded with a video camera. This camber development is analysed based on the viscous constitutive relations for porous based on the viscous constitutive relations for porous sintering bodies. The mismatched sintering kinetics of the two materials lead to the development of in-plane stresses in the two constituents of the composite structure as one material being constrained from composite structure as one material being constrained of the composite structure as one material being constrained from sintering by the other. The resulting camber during the consintering process is explained by the development of these stresses.

Effect of Mismatched Sintering Kinetics on Camber in a Low-Temperature Cofired Ceramic Package

This paper deals with the concept of microstructural engineering through donor and acceptor dopings within the grain and at the grain boundary of a polycrystalline semiconducting ceramic. These concepts are derived from an analysis of the “prebreakdown” and the “upturn” current-voltage characteristics of a ZnO varistor and from the construction of corresponding defect models as a function of donor and acceptor dopants at the grain and grain boundary. By using Li, Al, and Na as dopants, it is shown that the dopants can be grain or grain boundary specific in the ZnO microstructure and that they can act as donors, acceptors, or both, depending on the nature and concentration of dopants and their location on the host crystal lattice structure. In the case of the ZnO varistor, the grain and grain boundary properties can thus be tuned independently or concurrently by systematic engineering of the entire microstructure through defect dopings that are specific to the grain, grain boundary, or both. Following a detailed analysis of the defect models thus developed for the ZnO varistor, a set of ground rules are proposed for applying these concepts of donor and acceptor dopings at the grain and grain boundary to the general case of microstructural engineering in a polycrystalline semiconducting ceramic.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400019154

Microstructural engineering through donor and acceptor doping in the grain and grain boundary of a polycrystalline semiconducting ceramic

The ZnO varistor degradation has been attributed to the field-assisted, temperature-activated diffusion of interstitial zinc in the depletion layer. To improve stability, one approach is to reduce the formation of interstitials, and then further, to prevent their migration through empty interstitial sites. Based on this concept, an amphoteric dopant, such as Na or K, has been incorporated in the ZnO varistor grain boundary wherein a dopant is substituted both in the lattice and in the interstitial sites. A grain boundary defect model has been developed for this dual mode of substitution, with the dopant acting as an acceptor at the lattice site and as a donor at the interstitial site. Under these conditions, and given a desired neutrality range, the concentration of zinc interstitial is indeed shown to be reduced and stability greatly improved. The experimental data presented here validate the grain boundary defect model presented in this and in an earlier paper [J. Mater. Sci. 20, 3487 (1985)].

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400003046

Tapan K. Gupta

Papers

Application of Zinc Oxide Varistors

Application of Zinc Oxide Varistors

Effect of Mismatched Sintering Kinetics on Camber in a Low-Temperature Cofired Ceramic Package

Microstructural engineering through donor and acceptor doping in the grain and grain boundary of a polycrystalline semiconducting ceramic

Improved stability of the ZnO varistor via donor and acceptor doping at the grain boundary